Esterification of n-Butanol with Acetic Acid in the Presence of Heteropoly Acids with Different Structures and Compositions

The esterification reaction of n-butanol with acetic acid ([BuOH] : [HOAc] = 1 : 15 mol/mol; 55°C, 5% H₂O) was studied in the presence of tungsten heteropoly acids of the Keggin (H₃PW₁₂O₄₀, H₄SiW₁₂O₄₀, H₅PW₁₁TiO₄₀, H₅PW₁₁ZrO₄₀, and H₃PW₁₁ThO₃₉) and Dawson structure (-H₆P₂W₁₈O₆₂, H₆P₂W₂₁O₇₁(H₂O)₃, H₆As₂W₂₁O₆₉(H₂O), and H₂₁B₃W₃₉O₁₃₂). The reaction orders with respect to H₆P₂W₂₁O₇₁(H₂O)₃, H₃PW₁₂O₄₀, and H₆P₂W₁₈O₆₉are equal to 0.78, 1.00, and 0.97, respectively. It was found that the reaction rate depends on the acidity, as well as on the structure and composition of heteropoly acids. The H₂₁B₃W₃₉O₁₃₂heteropoly acid is most active, whereas the Keggin-structure heteropoly acids exhibit the lowest activities. Of the Keggin structure heteropoly acids, H₅PW₁₁ZrO₄₀exhibits the highest activity because of the presence of a Lewis acid site in its structure.     

Acid-catalyzed reactions of thiocyanates with camphene

The reactions of thiocyanates with camphene catalyzed by the heteropoly acids H₃PW₁₂O₄₀ and H₄SiW₁₂O₄₀ were studied. The optimal reaction conditions were found, and the formation of N-substituted thiocarbamates in comparatively high yields was established. It was shown that the heteropoly acids exhibit a higher catalytic activity than sulfuric acid.     

Hydrolysis of cellulose by the heteropoly acid H<Subscript>3</Subscript>PW<Subscript>12</Subscript>O<Subscript>40</Subscript>

The potential of heteropoly acid H₃PW₁₂O₄₀ to catalyze the hydrolysis of cellulose to glucose under hydrothermal conditions was explored. This technology could contribute to sustainable societies in the future by using cellulose biomass. A study to optimize the reaction conditions, such as the amount of catalyst, reaction time, temperature, and the amount of cellulose used, was performed. A remarkably high yield of glucose (50.5%) and selectivity higher than 90% at 453 K for 2 h with a mass ratio of cellulose to H₃PW₁₂O₄₀ of 0.42 were achieved. This was attributed to the high hydrothermal stability and the excellent catalytic properties, such as the strong Brønsted acid sites. This homogeneous catalyst can be recycled for reuse by extraction with diethyl ether. The results illustrate that H₃PW₁₂O₄₀ is an environmentally benign acid catalyst for the hydrolysis of cellulose.     

Trimetallic NiMoW/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> hydrotreating catalyst based on H<Subscript>4</Subscript>SiMo<Subscript>3</Subscript>W<Subscript>9</Subscript>O<Subscript>40</Subscript> mixed heteropoly acid

Trimetallic NiMoW/Al₂O₃ catalyst was prepared using mixed H₄SiMo₃W₉O₄₀ heteropoly acid of Keggin structure and nickel citrate. Bimetallic NiMo/Al₂O₃ and NiW/Al₂O₃ catalysts based on H₄SiMo₁₂O₄₀ and H₄SiW₁₂O₄₀, respectively, were synthesized as reference samples. The use of mixed H₄SiMo₃W₉O₄₀ heteropoly acid as an oxide precursor allows the tungsten sulfidation degree and the degree of promotion of active phase particles to be increased. The hydrodesulfurization activity is enhanced as compared to NiW/Al₂O₃ catalyst. The synergistic enhancement of the activity of the NiMo₃W₉/Al₂O₃ catalyst relative to the bimetallic analogs is probably caused by formation of new mixed promoted active sites for direct desulfurization.     

A Study of the Acid Properties of Structurally and Compositionally Different Heteropoly Acids in Acetic Acid

The acid properties of heteropoly acids of the following three structure types were studied by conductometry in acetic acid: Keggin (H₃PW₁₂O₄₀, H₃PMo₁₂O₄₀, H₄SiW₁₂O₄₀, H₃PW₁₁ThO₃₉; and H₅PW₁₁XO₄₀, where X(IV) = Ti or Zr), Dawson (-H₆P₂W₁₈O₆₂and -H₆P₂Mo₁₈O₆₂), and H₆P₂W₂₁O₇₁(H₂O)₃. These compounds are electrolytes that dissociate in only the first step of this solvent. The thermodynamic dissociation constants of the heteropoly acids were calculated by the Fuoss–Kraus method. The Hammett acidity functions H ₀of the solutions of H₅PW₁₁XO₄₀, H₃PW₁₂O₄₀, H₄SiW₁₂O₄₀, and H₆P₂W₂₁O₇₁(H₂O)₃in 85% acetic acid at 25°C were determined by the indicator method. All of the test heteropoly acids were found to be strong acids.     

Coupling of Phenol with Ketones in the Presence of Heteropoly Acids with Different Structures and Compositions

The reactions of phenol coupling with ketones MeCOR (R = CH₃, C₂H₅, C₃H₇, and C₄H₉) are studied in the presence of heteropoly acids with different structures and compositions in toluene solutions ([PhOH]/[MeCOR] = (2–8)/1 mol/mol; 50–70°C) with thioglycolic acid added as a promoter. The reaction rate depends on ketone and heteropoly acid, and the yield of bisphenols is as high as 24–72%. The reaction orders are 0.68, 0.77, and 0.97 with respect to H₆P₂W₂₁O₇₁, H₃PW₁₂O₄₀, and H₄SiW₁₂O₄₀, respectively, and the activation energies are 25.1, 21.0, and 20.6 kcal/mol, respectively. Heteropoly acids of the Dawson structure exhibited the highest activity.     

Kinetics and mechanism of the oxidation of arenes in the presence of heteropoly acids and palladium(II) complexes supported on silica gel

Pd(II) complexes and twelfth-series heteropoly acids (HPA) H₉[PMo₆V₆O₄₀] and H₃[PMo₁₂O₄₀] supported on silica gel oxidize benzene and toluene at 95°C. The formation of methyldiphenylmethane in the oxidation of toluene on HPA/SiO₂ and (PdCl₂−HPA)/SiO₂ catalysts, KIE>1 for the toluene/toluene-d₈ pair, and greater rate for toluene than for benzene are in accord with a one-electron transfer mechanism. L. M. Litvinenko Institute of Physical Organic and Coal Chemistry, National Academy of Sciences of Ukraine, 70 R. Lyuksemburg ul., Donetsk 340114, Ukraine. Translated from Teoreticheskaya i éksperimental'naya Khimiya, Vol. 35, No. 4, pp. 249–252, July–August, 1999.     

Catalytic properties of platinum-promoted acid cesium salts of molybdophosphoric and molybdovanadophosphoric heteropoly acids in the gas-phase oxidation of benzene to phenol with an O<Subscript>2</Subscript> + H<Subscript>2</Subscript> mixture

Acid salts Cs _x H_{3 + n ? x} PMo_{12 ? n} V _n O₄₀ (n = 0, 1, 2, or 3; x = 2.5 or 3.5) with coprecipitated or supported platinum were studied using thermogravimetry, IR spectroscopy, and temperature-programmed reduction. The thermal region of the full stability of these salts is limited by the decomposition temperature of the corresponding acid H₃PMo₁₂O₄₀ (～400°C) or H_{3 + n} PMo_{12 ? n} V _n O₄₀ (～300–350°C). The degree of reduction of heteropoly anions with hydrogen is regulated by temperature. Deeply reduced heteropoly anions (at 300°C) are slowly oxidized with oxygen with structure and composition regeneration. The states of molybdenum and vanadium on the surface of samples with coprecipitated platinum Pt_0.1-Cs_2.5H_0.5PMo₁₂O₄₀ (1) and Pt_0.1-Cs_2.5H_2.5PMo₁₀V₂O₄₀ (2), which were studied using XPS, correspond to reduced or reoxidized heteropoly anions in the bulk. Platinum metal particles of ～5 nm in size were observed in high-resolution TEM images obtained after the reduction and storage of sample 1 in air. A heteropoly compound forms two texture levels: spherical nanoparticles of 10–20 nm in size are collected in closely packed globules of 100–300 nm in size. Detailed texture studies, which were performed using nitrogen adsorption isotherms, demonstrated texture mobility under the ambient conditions. The cesium salts of the heteropoly acids were tested in the gas-phase oxidation of benzene to phenol with an O₂ + H₂ mixture at 180°. The effect of platinum concentration on the specific catalytic activity in the presence of deeply reduced heteropoly anions was monitored. The samples containing the salt Cs_2.5H_0.5PMo₁₂O₄₀ exhibited the highest activity in the formation of phenol. The introduction of vanadium into the heteropoly anion impaired the catalytic performance of both deeply and slightly reduced samples.     

Oxidation of isobutane catalyzed by vanadyl, copper and cesium substituted H3PMo12O40

Effects of Cs⁺, H⁺ and Cu²⁺ counterions in the vanadium containing heteropoly compounds Cs_xH_1-xVO[PMo₁₂O₄₀] and Cs_yH_0.5-yCu_0.25VO[PMo₁₂O₄₀] on the catalytic oxidation of isobutane and characterization by TGA, IR and ESR spectroscopies are reported. A high selectivity of 76% for methacrylic acid and methacrolein together has been obtained with Cs_0.75H_0.25VO[PMo₁₂O₄₀] catalysts at a reactivity of 5.3x10^-1 mmol/h cm³.     

A novel promoter, heteropoly acid, mediated chemo- and stereoselective sulfoxide glycosidation reactions

The chemo- and stereoselective glycosidations of sulfinylglycosides and alcohols using a heteropoly acid, H₃PW₁₂O₄₀, as a new promoter have been developed.     

Synthesis, characterization and studies on the third-order optical non-linearities of novel charge-transfer heteropoly complexes (C8H12N2)5H7PMo12O40 and (C8H12N2)3H3PMo12O40·5H2O

Summary Two novel charge-transfer (CT) heteropoly complexes, (C₈H₁₂N₂)₅H₇PMo₁₂O₄₀ (1) and (C₈H₁₂N₂)₃H₃-PMo₁₂O₄₀·5H₂O (2), prepared by reacting p-Me₂NC₆H₄NH₂ with the four-electron heteropoly blue H₇PMo₁₂O₄₀·12H₂O and heteropoly acid H₃PMo₁₂O₄₀· xH₂O, respectively, were characterized by elemental analysis, and u.v., i.r., XPS and e.s.r. spectroscopies. A sizable electron-transfer interaction occurs within the product molecules and the heteropoly anions retain their Keggin structure. Their third-order optical non-linearity coefficients were measured using the Z-scan technique at a concentration of 4.68 × 10⁻⁶ mol dm⁻³ for (1) and 2.79 × 10⁻⁶ mol dm⁻³ for (2), with I ₀ = 2.38 × 10¹³ w m⁻² and λ = 532nm. The |χ⁽³⁾| for (1) is 2.61 × 10⁻¹⁰ esu and |χ⁽³⁾| for (2) is 1.05 × 10⁻¹⁰ esu.     

A Keggin heteropoly acid as an efficient catalyst for an expeditious, one-pot synthesis of 1-methyl-2-(hetero)arylbenzimidazoles

The Keggin heteropoly acid, silicotungstic acid, H₄SiW₁₂O₄₀, has been demonstrated to be highly efficient for an expeditious, one-pot synthesis of 1-methyl-2-(hetero)arylbenzimidazoles from N-methyl-1,2-phenylenediamine and (hetero)aryl aldehydes in ethyl acetate at room temperature. The catalyst works equally well for N-phenyl-1,2-phenylenediamine.     

Cyclohexane oxidation with an O<Subscript>2</Subscript>–H<Subscript>2</Subscript> mixture in the presence of a two-component Pt/C–heteropoly acid catalyst and ionic liquids

Approaches to increase the efficiency of Pt/C–heteropoly acid catalyst in a liquid-phase oxidation of cyclohexane using an O₂–H₂ mixture were studied. It was shown that small additives of ionic liquid (BMImBr, Bu₄NBr, or Bu₄NHSO₄) significantly improve the catalytic effect of the Pt/C–H₃PMo₁₂O₄₀–CH₃CN system at 35°C, by slowing the rate of side reactions resulting in water formation, increasing the rate of oxygenate formation, and inhibiting their secondary oxidation reactions. The efficiency of H₂ consumption increases from 2 to 18–25%, while the selectivity of cyclohexane conversion is 92–98%. The substitution of one or two Mo(VI) ions by V(V) in the structure of the heteropoly acid decreases these parameters. In the presence of Bu₄NHSO₄, a Pt/C catalyst can be used many times. During the reaction, the heteropoly acid present in the solution is in a reduced state under the action of the reaction medium and undergoes reversible redox transformations. The nature of the catalytic action of the studied system is explained from the viewpoint of the effect of ionic liquids on the properties of a Pt/C catalyst in activating O₂, heteropoly molybdate chemistry, and the known mechanisms of the peroxide oxidation of hydrocarbons.     

Selective Esterification of Aliphatic Carboxylic Acids in the Presence of Aromatic Carboxylic Acids Over Monoammonium Salt of 12‐Tungstophosphoric Acid

Monoammonium salt of 12‐tungstophosphoric acid [(NH₄)H₂PW₁₂O₄₀] was found to be a practical and useful heterogeneous catalyst for an efficient and selective esterification of aliphatic carboxylic acids with alcohols in the presence of aromatic carboxylic acids. The heteropoly acid–based heterogeneous catalyst has the advantages of a simple workup procedure, water insolubility, and good activity.     

The enthalpies of formation and evolved gas detection

The standard molar enthalpies of formation of H₄SiW₁₂O₄₀·6H₂O (I), H₄SiW₁₂O₄₀·6DMF·H₂O (II), H₄SiW₁₂O₄₀·8DMSO·H₂O (III) have been determined. Thermodynamic cycles were designed, and the heat of reactions in the thermodynamic cycles were measured calorimetrically. The infrared spectra were compared with those of the heteropoly anion α-H₄SiW₁₂O₄₀ [1] and of the ligands DMF and DMSO. The evolved gas from the adducts was monitored by a quadrupole mass spectrometer at a heating rate of 16 deg·min^?1.     

Hydrocarbon oxidation with an oxygen-hydrogen mixture: Catalytic systems based on the interaction of platinum or palladium with a heteropoly compound

A series of studies of hydrocarbon oxidation by the O₂ + H₂ mixture in the presence of catalytic systems based on Pt or Pd and a heteropoly compound (HPC) is reviewed. The catalytic systems were prepared from Pd(II) complexes with the heteropoly tungstate anions PW₁₁O ₂₉ ^7? and PW₉O ₃₄ ^9? , the complex salt [Pt(NH₃)₄][H₂Mo₁₂O₄₀]₂ · 7H₂O, mixtures of H₂PtCl₄ or H₂PtCl₆ with H_{3 + n} PMo_{12 ? n} V _n O₄₀ (n = 0–3) heteropoly acids, or supported platinum dispersed in HPC solutions. The interaction of metal ions and particles with HPCs in the initial state and after thermal and redox treatments was investigated by NMR, IR spectroscopy, XPS, EXAFS, HREM, and TPR. The catalytic systems were tested in the liquid-phase oxidation of alkanes, cyclohexane, cycloalkenes, benzene, toluene, and phenol with the O₂ + H₂ mixture at low temperatures. Effective supported catalysts based on platinum nanoparticles associated with the redox-active HPCs H₃PMo₁₂O₄₀ and H₄PMo₁₁VO₄₀ were prepared for gas-phase benzene oxidation into phenol. The oxidation mechanism includes the interaction between dioxygen and platinum (or palladium) and the participation of the HPC in the formation of active oxygen species of radical nature.     

Selective Conversion of Cellobiose and Cellulose into Gluconic Acid in Water in the Presence of Oxygen,Catalyzed by Polyoxometalate‐Supported Gold Nanoparticles

Gold nanoparticles loaded onto Keggin‐type insoluble polyoxometalates (Cs_xH_3?xPW₁₂O₄₀) showed superior catalytic performances for the direct conversion of cellobiose into gluconic acid in water in the presence of O₂. The selectivity of Au/Cs_xH_3?xPW₁₂O₄₀ for gluconic acid was significantly higher than those of Au catalysts loaded onto typical metal oxides (e.g., SiO₂, Al₂O₃, and TiO₂), carbon nanotubes, and zeolites (H‐ZSM‐5 and HY). The acidity of polyoxometalates and the mean‐size of the Au nanoparticles were the key factors in the catalytic conversion of cellobiose into gluconic acid. The stronger acidity of polyoxometalates not only favored the conversion of cellobiose but also resulted in higher selectivity of gluconic acid by facilitating desorption and inhibiting its further degradation. On the other hand, the smaller Au nanoparticles accelerated the oxidation of glucose (an intermediate) into gluconic acid, thereby leading to increases both in the conversion of cellobiose and in the selectivity of gluconic acid. The Au/Cs_xH_3?xPW₁₂O₄₀ system also catalyzed the conversion of cellulose into gluconic acid with good efficiency, but it could not be used repeatedly owing to the leaching of a H⁺‐rich hydrophilic moiety over long‐term hydrothermal reactions. We have demonstrated that the combination of H₃PW₁₂O₄₀ and Au/Cs_3.0PW₁₂O₄₀ afforded excellent yields of gluconic acid (about 85 %, 418 K, 11 h), and the deactivation of the recovered H₃PW₁₂O₄₀–Au/Cs_3.0PW₁₂O₄₀ catalyst was not serious during repeated use.     

Über die thermischen Eigenschaften von Heteropolysäuren des Typs H3+n[PVnMo12−nO40] · x H2O (n = 0, 1, 2, 3) I. Thermogravimetrische,UV-VIS- und röntgenographische Untersuchungen

On the Thermal Behaviour of Heteropoly Acids of the Type H_3+n[PV_nMo_12?nO₄₀] · x H₂O (n = 0, 1, 2, 3). I. Thermogravimetry, UV-VIS, and X-ray Studies The thermal behaviour of pure and SiO₂-supported dodecamolybdophosphoric acid and its vanadium containing homologues was investigated using differential thermoanalysis, thermogravimetry, UV-Vis spectroscopy and X-ray. Up to appr. 450 K the crystal water free compounds are present. On raising the temperature the constitutional water is totally removed at appr. 650 K, without destroying the Keggin structure thereby. This is conducted from the fact that the initial compounds with the complete amount of crystal water are restorted by rehydration of the “anhydrides” in a water saturated atmosphere. Complete structural destruction proceeds only above 670 K. Highest thermal stability is achieved for the compound with n equalling 1. With decreasing concentration stability of the compounds supported on SiO₂ is strongly reduced. In contrast water vapour reacting with the products of total destruction, effects a partial reconstruction.     

双帽Keggin型杂多阴离子[H4As3Mo12O40]-和Keggin型杂多酸H3PM12O40 (M＝Mo, W)质子化的DFT研究

选用B3LYP方法在LanL2MB水平下, 对双帽α-Keggin型杂多阴离子[H₄As₃Mo₁₂O₄₀]^-的电子结构和质子的定位进行了密度泛函理论(DFT)研究. 结果表明, 双帽的形成大大影响了杂多阴离子[As₃Mo₁₂O₄₀]^5－的电子结构和性质, NBO分析显示参与成帽的三桥氧上的电子密度比双桥氧上的要大, 简单地从电荷密度来看, 质子将首先在三桥氧上定域成键, 但通过比较质子定域在几种桥氧上质子化稳定化能的大小, 发现[H₄As₃Mo₁₂O₄₀]^-中的四个质子将在八个双桥氧中的其中四个氧原子上定位, 而不是如文献中报道的在四个三桥氧上定域成键. 对杂多酸H₃PM₁₂O₄₀ (M＝Mo, W)中质子的定位也进行了理论计算并与文献进行了比较, 结果显示, H₃PMo₁₂O₄₀中质子是定位在双桥氧上; 而H₃PW₁₂O₄₀中质子将优先在双桥氧上定位, 但也可在端氧上定位; 这一结果与文献报道的相一致.     

Thermal Stability of Heteropoly Acids and Characterization of the Water Content in the Keggin Structure

The thermal stability of heteropoly acids of the Keggin type (H₄[SiMo₁₂O₄₀], H₃[PMo₁₂O₄₀], H₄[SiW₁₂O₄₀] and H₃[PW₁₂O₄₀]), being important new catalytic materials, was studied by DSC. Two groups of signals were observed: the low temperature endothermic peak group belongs to the water content, while the high temperature one is exothermic and indicates the thermal decomposition of the acids. The effect of microwave irradiation on the target compounds was also studied. The emphasis, however, was placed on the characterization of the water content of the acids. Several types of water can be classified and DSC curves provide additional information to explain the differences in the catalytic behavior. The study of the effect of heat treatment and the subsequent water absorption of the acids provided additional unique information concerning the pseudoliquid phase in the secondary structure of heteropoly acids. This revised version was published online in July 2006 with corrections to the Cover Date.